The Earth's water is always in movement, and
the water cycle, also known as the hydrologic cycle, describes the
continuous movement of water on, above, and below the surface of the
Earth. Since the water cycle is truly a "cycle," there is no beginning
or end. Water can change states among liquid, vapour, and ice at
various places in the water cycle, with these processes happening in
the blink of an eye and over millions of years. Although the balance
of water on Earth remains fairly constant over time, individual water
molecules can come and go in a hurry, but there is always the same
amount of water on the surface of the earth.
The water cycle has no starting or
ending point. The sun, which drives the water cycle, heats water in
the oceans. Some of it evaporates as vapour into the air. Ice and snow
can sublimate directly into water vapour. Rising air currents take the
vapour up into the atmosphere, along with water from
evapotranspiration, which is water transpired from plants and
evaporated from the soil. The vapour rises into the air where cooler
temperatures cause it to condense into clouds. Air currents move
clouds around the globe, cloud particles collide, grow, and fall out
of the sky as precipitation. Some precipitation falls as snow and can
accumulate as ice caps and glaciers, which can store frozen water for
thousands of years. Snow packs in warmer climates often thaw and melt
when spring arrives, and the melted water flows overland as snowmelt.
Most precipitation falls back into the oceans or onto land, where, due
to gravity, the precipitation flows over the ground as surface runoff.
A portion of runoff enters rivers in valleys in the landscape, with
stream flow moving water towards the oceans. Runoff, and ground-water
seepage, accumulate and are stored as freshwater in lakes. Not all
runoff flows into rivers. Much of it soaks into the ground as
infiltration. Some water infiltrates deep into the ground and
replenishes aquifers (saturated subsurface rock), which store huge
amounts of freshwater for long periods of time. Some infiltration
stays close to the land surface and can seep back into surface-water
bodies (and the ocean) as ground-water discharge, and some ground
water finds openings in the land surface and emerges as freshwater
springs. Over time, the water continues flowing, some to re enter the
ocean, where the water cycle renews itself.
